The invention concerns a data-reproduction device for recalling data that can be read out with an optical pick-up from the data-storage tracks of a recorded medium, whereby a beam of light is focused on the recorded medium by means of a focusing circuit and advanced along the data-storage tracks by means of a tracking circuit and whereby the beam of light is reflected from the recorded medium onto a photodetector that generates an electric data signal.
Devices of this type--compact-disk players, optico-magnetic equipment for recording and playback, and recording and playback equipment for DRAW-disk and videodisc players for example--have a optical pick-up consisting of a laser diode, several lenses, a prismatic beam divider, and a photodetector. The design and operation of an optical pickup are described on pages 209 to 215 of Electronic Components and Applications 6, 4 (1984).
The beam of light emitted from the laser diode is focused on the compact disk by lenses and thence reflected onto a photodetector. The data stored on the compact disk are obtained along with the actual value for the focusing circuit and tracking circuit from the signal emitted by the photodetector. The aforesaid reference calls the actual value for the focusing circuit the focusing error and the actual value for the tracking circuit the radial-tracking error.
The controls for the focusing circuit comprise a coil and an objective lens that travels through the coil's magnetic field along the optical axis. The focusing circuit keeps the beam of light emitted by the laser diodes constantly focused on the compact disk by moving the objective lens back and forth. The tracking circuit, which is also often called a radial drive mechanism, moves the optical pick-up radially in relation to the disk, positioning the beam on the spiraling data-storage tracks. In some equipment, the radial drive mechanism comprises what are called a coarse-adjustment mechanism and a fine-adjustment mechanism. The coarse-adjustment mechanism can for example be a spindle that moves the overall optical pick-up, consisting of the laser diode, the lenses, the prismatic beam divider, and the photodetector radially. The fine-adjustment mechanism tilts the beam of light radially, at a prescribed acute angle for example, and the beam can travel a short distance along the radius of the disk due to that motion alone.
To ensure unobjectionable recall of the data, which may represent both picture and sound in a videodisc player or sound alone in a compact-disk player for example, the beam of light must be not only precisely focused on the disk, but also precisely guided along the data-storage tracks.
The photodetector illustrated in the figure on page 213 of the aforesaid reference consists of four square photodiodes A, B, C, and D arrayed in a square. The beam of light focussed on the four photodiodes generates a data signal HF=AS+BS+CS+DS in the photodetector, wherein AS, BS, CS, and DS are the photoelectric voltages generated in photodiodes A, B, C, and D.
Data signal HF can be debased by contamination--dust and fingerprints for instance--and scratches on the disk, which deteriorate the reflectivity of the disk surface, the mirrored surface. Subjecting the compact-disk player to vibration will also interfere with data signal HF, causing the beam of light to leave the data-storage track being scanned and lose focus.
When data signal HF is subjected to interference resulting from vibrations, it is helpful to increase the amplification in the focusing and tracking circuits. Interference deriving from contamination on the surface of the disk on the other hand will be less evident when the amplification in the circuits is lower.
When the beam of light must skip ahead several tracks, to the beginning of a musical selection that the listener wants to hear for example, the number of tracks between the instant position of the beam and the target point, the beginning of the desired selection, is calculated. The tracking circuit then diverts the beam ahead to the beginning of the desired selection. In order to land precisely at the target point, however, the tracks that are to be skipped must be counted exactly.
When data-storage tracks are skipped, the upper envelope of data signal HF becomes sinusoidal. Its absolute amplitude is maximal when the beam shines on a track and minimal when it strikes the area between two tracks. The number of detected maxima will accordingly equal the number of tracks skipped.
As previously mentioned herein, the data signal can be debased by contamination on the surface of the disk and by vibrations, which can lead to errors in counting the maxima while the tracks are being skipped.